Uptake of Ba
2+ions by natural bentonite and CaCO
3:
A radiotracer, EDXRF and PXRD study
T. Shahwan,1 A. C. Atesin,1 H. N. Erten,1 A. Zararsiz2 1 Department of Chemistry, Bilkent University, 06533 Bilkent, Ankara, Turkey 2 Ankara Nuclear Research and Training Center, 06100 Besevler, Ankara, Turkey
(Received April 30, 2002)
Ba2+ uptake by natural bentonite, CaCO
3 in addition to a number of bentonite-CaCO3 mixtures with variable compositions as a function of pH and Ba2+ concentration was studied. Radiotracer method, EDXRF, and PXRD were used. The results of radiotracer experiments showed that the uptake of Ba2+ by CaCO
3 was larger than its uptake by natural bentonite samples, particularly at low initial concentrations of Ba2+ and higher pH values. This finding was supported by the EDXRF results. According to the sorption data, the apparent G° values of sorption were in the range –9H1 to –13H3 kJ/mol. The PXRD studies revealed the formation of BaCO3 upon sorption of Ba2+ on pure CaCO3 and on some of the bentonite-CaCO3 mixtures.
Introduction X-ray diffraction (PXRD) was used to elucidate the
structural changes that accompany the fixation of Ba2+ ions by the sorbent phases. Energy dispersive X-ray fluorescence (EDXRF) was applied to quantify the sorbed amounts of Ba2+ by the solids and the ‘depleted’ amounts of different cations in the solid matrix upon Ba2+ sorption.
The fact that radionuclides encountered in radioactive waste have a harmful impact on the biosphere makes investigation of the retardation of these radionuclides by various soil fractions an important issue. Barium isotopes include various radionuclides among which is 140Ba (T1/2=12.8 d). Although this radionuclide has a short half-life, it is particularly important because it is formed in high yield (6.24%) during the fission process.1
Experimental:
Natural bentonite samples of white color, relatively rich in calcium content, were obtained from the Turkish General Directorate of Mineral Research and Exploration (MTA). The samples originated from Giresun, situated in the Black Sea region of Anatolia. The particle sizes were <38 Pm. The CaCO3 used throughout the study was Merck extra pure CaCO3. Bentonite is a clay mineral with outstanding sorption
characteristics compared with other clay minerals. Its high cation exchange capacity, wide availability, thermal stability and favorable swelling behavior makes it one of the most appropriate clays suggested to be used as backfill material in the radioactive waste repositories.2
On the other hand, carbonate minerals are usually associated with the clays and consequently affect the sorption properties of those materials. Carbonate minerals play a major role in regulating the aquatic environments via precipitation, dissolution and sorption reactions. These reactions controlled by chemical processes are taking place at the interface between mineral lattice and bulk solution.3 The widespread
distribution of calcite (one form of CaCO3) in the Earth’s crust makes it one of the most important minerals to look at in terms of geochemically and environmentally important aqueous-metal partitioning reactions.4
Radiotracer experiments
The batch method was applied throughout the experiments. To each of a 30 mg solid sample placed in preweighed tubes, 3 ml of Ba2+ solution containing an
appropriate amount of 133Ba (T
1/2=10.7 y) radiotracer
was added. 133Ba tracer was used because of its long
half-life and well resolved prominent J-peak at 361 keV. The solid samples used in those studies were natural bentonite, CaCO3 and mixtures of both with bentonite percentages of 80, 60, 50, 40, and 20%. The initial concentrations of Ba2+ solution used in these
experiments were 1.0.10–2 and 1.0.10–6M. Tubes were
shaken at room temperature for a period of 48 hours in a temperature-controlled environment using a Nuve ST 402 water bath shaker equipped with a microprocessor thermostat. Following shaking, samples were centrifuged and 2 ml portions of the supernatant were counted using a 35 cm3 HPGe detector connected to a multichannel
PGT analyzer. Duplicate experiments were performed to check the reproducibility of the experimental data. This study is aimed of the investigation of the uptake
of Ba2+ ions by natural bentonite and CaCO
3, in addition
to mixtures of different proportions of the two solids. Since the carbonate surface is of a dynamic nature being to a large extent subject to pH variations, the experiments were performed using two sets; in one no pH control was attempted, and in the second the pH was kept constant at a high value (=9.0). The effect of initial Ba2+ concentration on sorption was investigated using
Tubes were vigorously hand-shaken prior to centrifugation to collect any liquid drops or solid particles adhering to the inner surface of each tube. The relative error in activity arising from adsorption by the inner tube surface was estimated to be less than 0.05, i.e., less than 5% of the radioactivity was removed from the sorption system by adsorption on the inner tube wall. The uncertainties in the measurements stemmed principally from those of counting statistics. Other error sources of less importance were those originating from weight and volume measurements. The error in the calculated Rd values was estimated to be less than r10% in all cases.
from 2 to 45 degrees in steps of 0.02 degree. The mineralogical compositions of the samples were determined using a search-match procedure.
EDXRF analysis
Energy dispersive X-ray fluorescence (EDXRF) analysis was carried out for the mineral samples before and after Ba2+ sorption. The analysis was done using a
multi-element analyzer (in the range 11Na to 92U), high performance Oxford ED2000 EDXRF system located at the Nuclear Research and Training Center of the Turkish Atomic Energy Authority, Ankara. The samples were analyzed for a total of 27 elements, using the Oxford Geological Majors and Tracers Method. XRF pellets were positioned in front of a Si (Li) detector and irradiated with X-rays originating from an Rh target. The detector was connected to a computer controlled system. The spectra were acquired and analyzed using Oxford Xpert Ease software. The tube power was 50 W and the maximum current was 1000 PA. Each sample was measured under five different conditions; very light element conditions, steel conditions, medium element conditions, heavy trace element conditions, very heavy element conditions. The weights of the samples and standards were approximately 2 g.
PXRD and EDXRF experiments
Throughout the sorption experiments the batch method was used and the initial concentration of Ba2+
solution was kept at 0.010M. The amounts of liquid and solid phases were fixed at 250.0 ml and 2.50 g, respectively. A magnetic stirrer was used for mixing and the period of contact was 48 hours at room temperature. Two sets of experiments were prepared. In the first set no pH control was attempted and the sorbent phases were obtained by mixing bentonite with CaCO3. The investigated compositions were 80%, 60%, 40%, and 20% bentonite in addition to pure natural bentonite and pure CaCO3 samples. The measured pH values at the end of mixing in this set of experiments were 4.03, 7.50, 7.69, 7.88, 8.59, and 9.89 for experiments where the solid phase consisted of 100%, 80%, 60%, 40%, and 20% bentonite in addition to pure CaCO3, respectively.
Results and discussion
Characterization of the solid samples
In the second set of experiments, the pH was kept at 9.0r0.1 for all samples. The pH was monitored regularly and adjusted whenever necessary, by adding an appropriate amount of 0.01M of NaOH. The solid phases used in this set of experiment were obtained by mixing bentonite and CaCO3 fractions in the same proportions as given previously. At the end of the mixing, the solid phases were filtered and dried overnight in an evacuated oven kept at 90 °C.
Natural bentonite and CaCO3 samples used in this study were characterized using EDXRF, FTIR, and PXRD. The chemical analysis of the untreated samples is given in Table 1. Natural bentonite, which contains a number of oxides with variable proportions, has a very high SiO2/Al2O3 ratio. This stems from the fact that the clay is a heterogeneous mixture of montmorillonite and silica minerals as discussed below. CaCO3 contains tiny amounts of Al and Fe oxides as impurities.
PXRD analysis
PXRD patterns of natural bentonite, CaCO3, and a mixture of both are given in Fig. 1. Natural bentonite samples are composed of montmorillonite, characterized by the major (001) peak occurring at d001=15.15, in addition to cristobalite (low-type), an allotrope of silica that has the prominent (101) peak at d101=4.05. The PXRD diagram of CaCO3 reveals that it is composed of calcite and aragonite. Calcite and aragonite have, among other peaks, the characteristic (104) and (111) peaks occurring at d104=3.03 and d111=3.39, respectively. Powder X-ray diffraction (PXRD) analysis was
carried out using a Rigaku Miniflex model instrument. Samples of natural and Ba-sorbed bentonite, CaCO3 and bentonite-CaCO3 mixtures were analyzed in the powder form. The source consisted of unfiltered Cu KD radiation, generated in a tube operating at 30 kV and 15 mA. Spectra were recorded with 2 T values ranging
Table 1. Chemical composition of natural bentonite, CaCO3, and their different mixtures obtained by EDXRF technique (in percent composition by weight
Constituent 100% bentonite 20% CaCO3 40% CaCO3 50% CaCO3 60% CaCO3 80% CaCO3 100% CaCO3
+ + + + +
80% bentonite 60% bentonite 50% bentonite 40% bentonite 20% bentonite
MgO 0.97 0.95 0.62 0.76 0.36 0.26 0.00 Al2O3 9.71 8.57 5.94 6.35 3.87 2.01 0.24 SiO2 85.52 64.65 41.84 43.05 25.45 9.92 0.00 K2O 0.26 0.17 0.10 0.08 0.07 0.00 0.00 CaO 1.15 23.97 49.73 48.28 68.97 87.06 98.65 TiO2 0.26 0.23 0.21 0.19 0.15 0.06 0.00 MnO 0.02 0.02 0.02 0.01 0.01 0.01 0.01 Fe2O3 1.60 1.43 1.31 1.06 0.91 0.43 0.04 Others <0.51 <0.10 <0.23 <0.22 <0.21 <0.25 <0.06
Fig. 1. PXRD diffractograms of natural bentonite (a), CaCO3 (b) and a mixture of bentonite and CaCO3 (50:50) (c). Mont.: montmorillonite, Cris.: cristobalite, Arag.: aragonite, Calc.: calcite
The IR spectra of natural bentonite, CaCO3, and a mixture of both are given in Fig. 2. The presence of montmorillonite (Fig. 2a) component in natural bentonite can be identified from the features occurring at 3623, 912, and 842 cm–1. The broad peak at 3625 cm–1 forms
an envelope for a wide range of AlAlOH and AlMgOH environments, the latter resulting from the isomorphous substitution of Mg for Al in the octahedral sheet of the clay. More characteristic of montmorillonite are the well-resolved deformation bands at 915 cm–1 (AlAlOH)
and 840 cm–1 (AlMgOH). The features at 1423-1576,
872, 857 and 713 cm–1 are diagnostic for CaCO 3
(Fig. 2b). These bands are indicative that CaCO3 samples are composed of calcite, and aragonite,5 in line
with the PXRD characterization.
Table 3. Relative concentration (in wt%) of sorbed Ba2+ on different bentonite–CaCO3 mixtures obtained from EDXRF measurements,
no pH control (a) and pH 9.0 (b)
Solid phase composition (a) (b) 80% Bentonite + 20% CaCO3 4.59 7.31 60% Bentonite + 40% CaCO3 4.56 9.01 40% Bentonite + 60% CaCO3 4.67 11.71 20% Bentonite + 80% CaCO3 9.50 12.66
The apparent Gibbs free energy values of sorption,
'G°, were calculated by:
'G° = –RT ln Rd (2)
Here Rd is approximated to be the equilibrium constant. Actually Rd is a function of the initial concentration and as such leads to changes in the corresponding apparent 'G° values. In order to obtain
'G° values representative over the entire concentration
ranges, arithmetic averages of apparent 'G° values were calculated for each solid composition at room temperature (295 K). The results are given in Table 2. The negative 'G° values in all cases indicate that the Ba2+ sorption is spontaneous. The magnitudes of 'G° in
the 8–16 kJ/mol range indicate that the sorption mechanism is mainly of ion exchange type.6
Radiotracer studies
The experimental data obtained from radiochemical studies were expressed in terms of the ‘distribution ratio’, Rd, given as:
R A A A V M d q 1 1 (1)
where A° is the initial count rate of solution before sorption, (cps)/ml; A1 is the count rate of solution after sorption, (cps)/ml; V is the volume of solution, ml; and
M is the weight of solid material, g.
EDXRF studies
The EDXRF technique enabled the chemical analysis of different samples prior to and following sorption. According to EDXRF findings, the weight percentages of sorbed Ba2+ increased from 0.043 for the samples of
100% bentonite up to 29.09 for the ones composed of 100% CaCO3. For the bentonite-CaCO3 mixtures, the weight percentages of Ba2+ increased, as the samples
became richer in CaCO3. This trend is observed for both sample series (a, b) as shown in Table 3. The table shows also that the increase in pH resulted in an increase in sorbed Ba2+ amounts for all samples.
The Rd values for two different initial concentrations of Ba2+ are given in Table 2. While there is no clear trend in the dependence of Rd values on CaCO3 content of the solid mixtures at the higher initial Ba2+ concentration, an increase in Rd is obvious as CaCO3 content increases at the lower initial Ba2+ concentration. The Rd values reported in Table 2 correspond to the arithmetic mean of the data obtained from duplicate experiments with a relative error ranging within r2% to 10%.
The amounts of different constituents of the solid matrix prior to and following the sorption of Ba2+ were
analyzed. According to the results, a depletion of Ca, Mg, and K is observed upon Ba2+ sorption. In order to
enable the comparison of the quantities of these elements prior to and following sorption, they were normalized to the Al+Si oxides. Both Al and Si are ‘skeletal’ elements that can be assumed not to take part in the exchange process. For the samples composed of 100% bentonite, the dominating exchanged cation was Ca2+, for which
the CaO/(Al2O3+SiO2) ratio changed from 0.0112 to 0.0025. For CaCO3, this ratio changed from 429 to 75. For samples composed of different bentonite and CaCO3 proportions, the results are given in Table 4. The largest depletion is shown by CaO, which is contained in larger quantities compared with the others.
Table 2. Rd values and the corresponding averaged apparent G° (calculated at T = 295 K) values for two different initial concentrations of Ba2+ 10–2M Ba2+ (a) and 10–6M Ba2+ (b)
CaCO3, % Rd, ml/g G°, (a) (b) kJ/mol 0 20 43 –8 H 1 20 35 45 –9 H 1 40 22 58 –9 H 1 50 29 70 –9 H 1 60 20 249 –10 H 3 80 29 326 –11 H 3 100 45 649 –13 H 3
Table 4. Variation in the relative concentrations of MgO, CaO, and K2O normalized to Al2O3+SiO2 content, obtained from EDXRF analysis of: samples prior to sorption (a), Ba-sorbed samples (no pH control) (b), and Ba-sorbed samples (pH 9.0) (c)
Element/(Al+Si)
Element Sample 80% bentonite 60% bentonite 40% bentonite 20% bentonite
+ + + +
20% CaCO3 40% CaCO3 60% CaCO3 80% CaCO3
MgO (a) 0.013 0.012 0.012 0.022 (b) 0.012 0.012 0.008 N.D. (c) 0.013 0.011 0.012 0.011 CaO (a) 0.32 1.04 2.35 7.30 (b) 0.20 0.63 2.10 4.40 (c) 0.20 0.59 1.38 4.67 K2O (a) 0.0023 0.0021 0.0020 N.D. (b) 0.0018 0.0019 0.0014 N.D. (c) 0.0017 0.0015 0.0015 N.D. N.D.: Not detected.
Fig. 3. PXRD diffractogram of Ba-sorbed CaCO3 (a), and PXRD diffractogram of Ba-sorbed natural bentonite (b)
PXRD studies 'd001=1.77 Å for Zn2+) and attributed to changes in the
hydration state of the interlayer spacing of the clay upon sorption.7
The PXRD diagrams of sorbed bentonite and Ba-sorbed CaCO3 are given in Fig. 3. The sorption of Ba2+
on CaCO3 is marked by the formation of BaCO3 features (Fig. 3a) accompanied by a reduction in the other peak intensities. The sorption of Ba2+ on natural bentonite
(Fig. 3b) leads to a significant intensity reduction of the montmorillonite (001) peak and to a slight change in its peak position from d001=15.2 Å to d001=13.3 Å, corresponding to 'd001=1.9 Å. Peak position changes had been reported for sorption of Pb2+ and Zn2+
on Ca-montmorillonite ('d001=2.64 Å for Pb2+,
PXRD was also used to analyze the uptake of Ba2+
by mixtures of bentonite and CaCO3 with variable compositions. The diffractograms of the samples prepared without any pH control and those prepared while pH was fixed at 9.0 are given in Figs 4a and 4b, respectively. For the sorption experiments carried out without any pH control, where the pH changed from 4.03 to 9.89 depending on the CaCO3 content, BaCO3 formation was observed for mixtures containing at least 60% CaCO3 by composition as shown in Fig. 4a.
Fig. 4. PXRD diffractogram of Ba-sorbed mixtures of bentonite and CaCO3 (no pH control) (a), and PXRD diffractogram of Ba-sorbed mixtures of bentonite and CaCO3 (pH 9.0) (b)
When the pH was kept constant at 9.0r0.1, the precipitate formation was observed for samples having 20% CaCO3 or higher as given in Fig. 4b. The increase in pH is expected to enhance the role of the CaCO3 fractions in Ba2+ fixation. The pH has minimal effects on sorption on bentonite due to the low ‘zero point of charge (ZPC)’ for such clays, which is generally below ~2.5 pH units.8 The increased role of CaCO3 fractions in Ba2+ sorption is reflected in larger amounts of Ba2+ being sorbed when the pH was increased as was previously stated based on the EDXRF measurements.
fixation is verified by the formation of BaCO3. For a given initial Ba2+ concentration, BaCO3 formation is dependent on the amount of CaCO3 present in the solid phase in addition to pH value. BaCO3 presence was detected for bentonite samples containing at least 60% CaCO3 by mass when no pH control was carried out. When the pH was raised to 9.0 and kept constant, BaCO3 formation appeared for bentonite samples containing 20% CaCO3 by mass or larger.
References Conclusions
1. K. H. LIESER, Radiochim. Acta, 70/71 (1995) 355.
2. M. TAKAHASHI, M. MUROI, A. INOUE, M. AOKI, M. TAKIZAWA, K. ISHIGURE, N. FUJITA, Nucl. Techn., 76 (1987) 221.
The uptake of Ba2+ by bentonite is enhanced as the quantity of CaCO3 incorporated in the clay increases, particularly, at lower Ba2+ concentration and higher pH values of the sorption system. The sorption of Ba2+ is spontaneous and is accompanied by a depletion of K+, Mg2+ and Ca2+ ions originally present within the solid phases. The interlayer spacing of bentonite seems to be affected by Ba2+ sorption, as the montmorillonite (001) peak position showed some changes, the thing that might be attributed to a change in the hydration state of the interlayer space of the clay. The role of CaCO3 in Ba2+
3. P. V. CAPPELLEN, L. CHARLET, W. STUMM, P. WERSIN, Geochim. Cosmochim. Acta, 57 (1993) 3505.
4. N. X. MICHAEL, F. HOCHELLA Jr., G. BROWN Jr., A. PARKS, Geochim. Cosmochim. Acta, 60 (1996) 2801.
5. M. J. WILSON, Clay Mineralogy: Spectroscopic and Chemical Determination Methods, Chapman & Hall, London, 1994. 6. F. HELFERRICH, Ion Exchange, Mc Graw Hill, New York, 1962. 7. M. AUBOIROUX, P. BAILLIF, J. C. TOURAY, F. BERGAYA, Appl.
Clay Sci., 11 (1996) 117.
8. J. I. DREVER, The Geochemistry of Natural Waters, Prentice-Hall, New Jersey, 1982.
